Transfusion of red blood cells (RBC) is the only clinically effective therapeutic approach for treating oxygen transport deficits (i.e. blood loss in surgical interventions and anaemia). However potential shortage of transfusable RBC has been predicted for the near future as a result of an imbalance between supply and demand due to aging population, an increase in the transmission of infectious diseases, limited compatibility of stored stocks, and the requirement for rare blood groups [1, 2]. This situation has a direct impact in Public Health and it has consequently spurred the development of novel technologies for the generation of blood substitutes. The candidate products for human use should be safe, display adequate profiles for the uptake, transport and delivery of oxygen, a prolonged half-life in the bloodstream, stability at room temperature that would facilitate cost-effective storage, and they must be obtained under Good Manufacturing Practice (GMP) quality standards. In vitro production of RBCs [3–5] from hematopoietic stem/progenitor cells (HSC) [6, 7], embryonic stem (ES) cells , or induced pluripotent stem (iPS) cells  under controlled culture conditions offers a potential solution to overcome this medical and social issue. However mass production of RBC has not been attained yet. The attractiveness of developing bioprocesses for ex vivo production of RBC also resides in the fact that enucleated cells pose no risk of tumorigenicity (no matter whether RBC are derived from immortalized or pluri-/multipotent cells) and, therefore, they can be transfused without hazard into the recipient. Enucleated RBCs can be selected by size (e.g., by filtration), and impurities of nucleated cells can be eliminated by irradiation without affecting the structure and function of RBCs. Indeed, such irradiation is routinely used before transfusion in order to eliminate any remaining lymphocytes. Besides, transplantation of progenitor cells requires compatibility for major histocompatibility antigens , but this is not the case for enucleated RBCs, which only require the compatibility of ABO and RhD blood phenotypes.
Given the potential of stem cells to recapitulate erythropoiesis in vitro under controlled conditions in standard T-flask cultures, we transferred such methodology into stirred tank bioreactors, as the first step towards scaling the bioprocess up to the production of clinically relevant doses. Furthermore, we compared the characteristics of the cells produced in bioreactors to those obtained from traditional manual cultures.